One class of well-known moisture sensors for sensing humidity and moisture condensation includes moisture sensors comprising a moisture sensitive film having conductive particles dispersed in a hygroscopic polymer as disclosed in JP-A 99740/1983, 170755/1984, 250241/1985, 21052/1987, and 249813/1994.
Among these, JP-A 21052/1987 discloses the use of polyvinyl alcohol (PVA, also known as POVAL) as the hygroscopic polymer. This sensor is insufficient in sensitivity and condensing/drying cycle performance. As an improvement over this sensor, JP-A 249813/1994 discloses a polyvinyl alcohol having a degree of saponification of 60 to 95%, carbon black having a specific surface area of 80 to 800 m.sup.2 /g as the conductive particles, and a moisture sensitive film having a thickness of less than 5.0 .mu.m.
The condensing/drying cycle performance is not satisfactorily improved by using such a partially saponified polyvinyl alcohol. Polyvinyl alcohols including partially saponified ones must be crosslinked when they are actually used in moisture sensors. After the crosslinking treatment, however, some properties including condensing/drying cycle performance are significantly exacerbated, particularly in the case of JP-A 249813/1994.